Baseball-Reference.com has a nice little stat called OPS+, which compares the players OPS to the league-adjusted OPS for each season played. An OPS+ of 100 is league average. Above average is over 100; below average is less than 100. So, this stat does exactly all the things you claim would be fair to assess players from different eras. Franco has some very nice offensive seasons in his career, with 5 of them over 120 OPS+. His career OPS+ is 112; Youk's is 103 after 2 years, with a 115 last year. I agree with you. If Youk can go on a stretch where he goes 137, 121, 146, 95 (injury shortened season), 118, 137, 122 like Franco did from '89 to '96, then he surpass anyone's expectations of him right now.